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Hightemperatureoxidationresistanceof(Ti,Ta)(C,N)-basedcermets

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ErnestoChicardi

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UniversidaddeSevilla

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FranciscoJ.Gotor

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1

High temperature oxidation resistance of (Ti,Ta)(C,N)-based cermets

E. Chicardi*1,2, J.M. Córdoba1,3 and F. J. Gotor1

1Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Américo Vespucio 49, 41092

Sevilla, Spain.

2Departamento de Ingeniería Metalúrgica y de Materiales, Universidad Técnica

Federico Santa María, Valparaíso 1680, Chile.

3Departamento de Química Inorgánica, Universidad de Sevilla (US), Calle Prof. García

González 1, 41012, Sevilla, Spain

Abstract

Cermets based on titanium-tantalum carbonitride were oxidized in static air

between 800 ºC and 1100 ºC for 48 h. The thermogravimetric and microstructural study

showed an outstanding reduction in the oxidation of more than 90% when the Ta

content was increased. In cermets with low Ta content, the formation of a thin

CoO/Co3O4 outer layer tends to disappear by reacting with the underlying rutile phase,

which emerges at the surface. However, in cermets with higher Ta content, the

formation of an external titanate layer, observed even at a low temperature, appears to

prevent the oxygen diffusion and the oxidation progression.

Keywords: A. Ceramic matrix composites; A. Cobalt; A. Titanium; B. SEM; B. X-ray

diffraction; C. Oxidation.

* Corresponding Author. Tel: +34 954489217. Fax: +34 954460165. E-mail:

echicardi82@gmail.com / ernesto.chicardi@icmse.csic.es

1. Introduction.

The materials used in various applications, such as cutting tools (rock and earth

drilling tools), drawing and sheet metal-forming tools and dies, wear parts (nozzles,

plunges, paper cutters, etc.) and other structural components (mechanical seals, boring

2

bars, etc.), can encounter extreme conditions (high temperature, pressure, and an

oxidizing atmosphere, among others) during use. For this reason and because of

constantly increasing industrial demands, the search for novel materials and designs

with improved performance remains a challenge. Materials for these applications are

expected to possess not only high abrasive and impact wear resistance but also excellent

high temperature corrosion or/and oxidation resistance. Oxidation in non-oxidic

structural materials is a major disadvantage because it typically controls the operational

properties. Oxidation can reduce ductility, tensile strength and erosion and corrosion

resistance, among other properties [1-3]. Oxidation is not always a slow process and can

progress significantly over a short service time in certain practical applications [4].

Hard materials used in the metal working industry for high-speed machining must

withstand a broad range of thermal, mechanical and chemical stresses. These materials

are primarily based on tungsten carbide (cemented carbides) or titanium carbonitride

(cermets). The desired properties can be achieved through complex alloying with other

transition metals, such as Ta and Nb [5-7]. In this context, (Ti,Ta)(C,N)-based cermets

have been successfully utilized as heat-resistant structural materials, including wear

parts and, especially, cutting tools in high-speed finishing and semi-finishing

applications due to their excellent physical and chemical properties, such as a favorable

hot hardness, excellent wear resistance, perfect chemical stability, low friction

coefficient to metals and great thermal deformation resistance [8-15]. However, while in

service, (Ti,Ta)(C,N)-based cermets are often exposed to elevated temperatures in

oxidizing environments [16]; therefore, their oxidation behavior plays a key role in

cutting performance and tool life.

In a previous study [17], it was determined at a fixed temperature of 900 ºC that

the oxidation resistance of the (Ti,Ta)(C,N)-based cermets was significantly dependent

on the Ta content. The goal of the present work is to report a systematic study of the

outstanding oxidation improvement of TixTa1-xC0.5N0.5-Co cermets over a wide

temperature range (800 ºC-1100 ºC) to address the lack of available experimental data

in the literature on this topic. This study also attempts to provide new insights into the

key role played by Ta in the excellent oxidation resistance of cermets. Note that the

temperatures studied cover the maximum cutting tool work temperatures for the most

demanding processes [18]. It must be considered that cutting tool oxidation at these

3

temperatures is more important than the mechanical abrasion in determining the tool life

in many cases.

2. Experimental.

Five cermets with nominal compositions of 80 wt% TixTa1-xC0.5N0.5 – 20 wt% Co

and variable Ta atomic content, x = 1, 0.99, 0.95, 0.90 and 0.80 (labeled as Ti100,

Ti99Ta1, Ti95Ta5, Ti90Ta10 and Ti80Ta20, respectively), were synthesized by the

mechanochemical process called the Mechanically Induced Self-Sustaining Reaction

(MSR) from Ti, Ta and C powder mixtures and N2 gas. A powder mixture containing

46.5 g elemental Ti (99% purity, <325 mesh, Strem Chemicals), Ta (99.6% purity, <325

mesh, Alfa-Aesar), C (as graphite, <270 mesh, Fe < 0.4%, Merck) and Co (99.8%

purity, <100 mesh, Strem Chemicals), together with 13 tempered steel balls (d = 20

mm, m = 32.6 g), were put into a tempered steel vial (300 ml in volume) and milled

using a modified planetary ball mill (P4, Fritsch) at a spinning rate of 400 rpm under 6

atm N2 gas (H2O and O2 < 3 ppm, Air Liquide). The planetary ball mill enabled the

MSR reactions to be monitored by continuously measuring the pressure inside the vial.

When the MSR reaction associated with the synthesis of the TixTa1-xC0.5N0.5 carbonitride

solid solution phase occurs, the temperature increases due to the release of heat from the

exothermic formation reaction, which consequently increases the total pressure. The

ignition time, i.e, critical milling time required to induce the MSR process, can then be

determined from the spike in the recorded time-pressure data. After ignition, the milling

is continued to ensure full conversion and homogenization. The ignition time was about

45 min for all cermets and the total milling time was 75 min.

Subsequently, the powdered cermets were shaped using a uniaxial press (2 tons, 5

min) and compacted by cold isostatic pressing (200 MPa, 10 min). The green bodies

were then sintered at temperatures between 1450 ºC and 1550 ºC, optimized for each

Ta/Ti ratio [19], for 60 min under a flowing Ar (H2O < 3 and O2 < 2 ppm, Linde)

atmosphere in a horizontal tubular furnace (IGM1360 model No. RTH-180-50-1H,

AGNI) to obtain cylindrical cermets 13 mm in diameter and 9 mm in height. After

sintering, the cermets consisted of the mentioned carbonitride phase (TixTa1-xC0.5N0.5)

and two Ti-Ta-Co intermetallic compounds, namely, TixTa1-xCo2 and TixTa1-xCo, acting

4

as binder instead of the initial Co. A detailed description of the microstructure and

properties of cermets can be found in a previously published study [19].

To perform the oxidation experiments, parallelepiped specimens of these five

sintered cylinder cermets with similar dimensions (3±0.1 mm x 4±0.1 mm x 5±0.1 mm)

and surface area (94±0.5 mm2) were obtained by cutting from the center part of the

sintered body. The surfaces of cermets were subjected to successive grinding and

polishing steps (final step with 3 m diamond powder suspension). Finally, the

specimens were ultrasonically cleaned with ethanol. The oxidation tests were performed

in static air under isothermal conditions from 800 ºC to 1100 ºC (heating rate of 20

ºC/min and free cooling) for 48 h. The weight gain due to oxidation was continually

monitored using a CI Robal electrobalance (C.I. Electronics Ltd.) with a maximum

weight allowed of 5 g and a sensitivity of 1 μg; the balance was attached to the support

frame of a high-temperature vertical furnace (Severn Thermal Solutions Ltd.) designed

for use up to 1500 ºC. The specimens were placed on a platinum wire coil to maximize

the contact of the surfaces with the surrounding atmosphere.

X-ray diffraction (XRD) diagrams were collected on the surfaces exposed to

oxidation to get information about the phases present in the oxide scale of the oxidized

cermets. XRD patterns were also obtained once the oxidized specimens (bulk + oxide

scale) were crushed and reduced to powder by hand-grinding to obtain information on

the extent of oxidation. A PANalytical X’Pert Pro instrument equipped with a θ/θ

goniometer, a Cu K radiation source (40 kV, 40 mA), a secondary K filter and an

X’Celerator detector was used. The diffraction patterns were collected from 20º to 80º

(2θ) in a step-scan mode at a step of 0.02º and a counting time of 275 s/step and were

compared with those in the PDF-4+ database from the International Centre for

Diffraction Data (ICDD). Lanthanum hexaboride powder (Standard Reference Material

for powder diffraction 660b, NIST) was used to calibrate the positions of the diffraction

lines. Lattice parameters were calculated from the complete set of peaks in the XRD

pattern using the DICVOL04 software.

The polished cross sections of the oxidized specimens were examined via

scanning electron microscopy (SEM) performed on a Hitachi S-4800 SEM-FEG

microscope coupled with an X-ray energy dispersive spectrometry (XEDS) detector

5

(Quantax-EDS, Bruker Corporation), which was used for the chemical

semiquantification of the ceramic, binder and oxidized phases.

3. Results and Discussion.

3.1. Dependence of oxidation on temperature and tantalum content.

The weight gain normalized to the surface area of the material (per unit area) after

48 h of oxidation for all of the temperatures and cermets is shown in figure 1. A

significant decrease in the extent of oxidation is clearly observed for all of the

temperatures when the Ta content is increased. Cermet without Ta (Ti100) exhibited a

non-negligible oxidation (18.6 mg/cm2), even at the lowest oxidation temperature tested

(800 ºC). However, cermet Ti90Ta10 with a higher Ta content required a substantially

higher temperature (1100 ºC) to produce a similar oxidation level (19.5 mg/cm2). At

1100 ºC, cermet Ti80Ta20 showed a low oxidation of 9.5 mg/cm2, which was lower

than that of cermet Ti100 at 800ºC. At 800 ºC, the oxidation of cermets Ti95Ta5,

Ti90Ta10 and Ti80Ta20 was practically imperceptible. In contrast, cermets Ti100 and

Ti99Ta1 were extensively oxidized at 1100 ºC.

To quantify the improvement in the oxidation resistance of cermets when Ti is

partially substituted by Ta, the percentage reduction of the weight gain per unit area for

each cermet at each temperature using the Ti100 cermet as reference is presented in

table 1. By replacing only 1 at% of Ti with Ta (Ti99Ta1), oxidation was reduced by

67.2% at 800ºC. This improvement remained high for Ti99Ta1 until 1100 ºC (46.3%).

At this high temperature, the enhancement of oxidation resistance increases with

increasing Ta content, reaching a remarkable 92.8% for Ti80Ta20. An improvement of

at least 90% is also achieved in cermets Ti95Ta5 and Ti90Ta10 but at lower

temperatures of 900 ºC and 1000 ºC, respectively.

To corroborate the thermogravimetric results and to analyze the phases formed

during the oxidation tests, XRD patterns of oxidized cermets reduced to powder are

compared in figure 2. The XRD pattern corresponding to oxidized Ti100 cermet at a

low temperature (800 ºC) clearly shows the presence of rutile TiO2 (tetragonal structure,

P42/mnm, 136) due to the cermet oxidation. The intensity of the TiO2 reflections

increases with the oxidation temperature as the oxidation progresses. Moreover, new

6

reflections that can be assigned to cobalt methatitanate (CoTiO3, rhombohedral

structure, R-3, 148) are detected from 900 ºC. The existence of this Co-containing phase

is due to the presence of Co in the binder phase, which is also oxidized. Simultaneously,

the intensity of the original ceramic and binder phases is considerably reduced. At 1000

ºC, the unoxidized phases are only slightly detected in the XRD patterns, suggesting a

high degree of oxidation, in accordance with the thermogravimetric results. At 1100 ºC,

the XRD pattern is practically dominated by the TiO2 rutile reflections. Note that as

described in the literature [19], the binder phase in cermets developed by MSR was

composed of a mixture of TiCo (cubic structure, Pm-3m, 221) and TiCo2 (cubic

structure, Fd-3m, 227) intermetallic phases. Therefore, the presence in the oxidized

specimen XRD patterns of reflections corresponding to a α-Co alloy (cubic structure,

Fm-3m, 225) implies the preferential oxidation of Ti vs Co in the binder phase.

The XRD patterns of oxidized Ti99Ta1 cermet (figure 2) show a similar trend to

that of Ti100 cermet as the oxidation progresses with temperature. However, the

improvement in the oxidation resistance by substituting only 1 at% of Ti with Ta is

evidenced by the delay in the evolution of oxidized phases, corroborating the

thermogravimetric results in figure 1. A comparison of the XRD patterns of the

oxidized cermets reveals that the patterns are similar but shifted by 100 ºC, e.g., XRD

patterns at 900 ºC, 1000 ºC and 1100 ºC for Ti99Ta1 cermet are equivalent to those at

800 ºC, 900 ºC and 1000 ºC for Ti100 cermet, respectively. This result indicates that

oxidation was deferred by approximately 100 ºC.

For Ti95Ta5 cermet, the XRD patterns in figure 2 indicate that the oxidation

remained at a low level until 1100 ºC in which the reflections of the original unoxidized

phases were still clearly visible. The XRD pattern of Ti95Ta5 cermet at 1100 ºC was

similar to that of Ti99Ta1 and Ti100 cermets at 900 ºC and 800 ºC, respectively,

showing again the positive effect of Ta substitution on oxidation resistance. In this case,

when Ti was replaced with 5 at% of Ta, the oxidation was delayed by approximately

300 ºC compared with cermets without Ta.

For Ti90Ta10 and Ti80Ta20 cermets, with the highest Ta contents of 10 and 20

at%, respectively, the XRD patterns in figure 2 show an even greater oxidation

resistance. The XRD patterns of Ti80Ta20 cermet are dominated by the unoxidized

7

phases over the entire temperature range studied. Only at 1000 ºC, the most intense

reflections of the rutile structure begin to emerge from the background signal.

Note that single tantalum oxides, such as Ta2O5 or TaO2, were not detected in the

XRD patterns of oxidized cermets (figure 2). However, displacements toward the lower

2θ angles were observed in the reflections corresponding to the rutile structure when the

Ta amount was increased. This effect is illustrated in figure 3 in which the XRD

patterns over the range of the (110) and (211) reflections for TiO2 and TaO2 are shown

for the five different cermets oxidized at 1100 ºC. The observed shift is equivalent to an

increase in the unit cell parameters as shown in table 2 and suggests that Ta is

introduced into the TiO2 lattice, resulting in the formation of a TixTa1-xO2 solid solution

with rutile-type structure (tetragonal structure, P42/mnm, 136).

3.2. Surface characterization of the oxidized cermets.

To obtain precise information on the nature of the oxide scale formed in oxidized

cermets, XRD patterns of the surfaces exposed to oxidation were recorded and are

shown in figure 4. In the XRD pattern of Ti100 cermet oxidized at 800 ºC, CoO (cubic

structure, Fm-3m, 225) and Co3O4 (cubic structure, Fd-3m, 227) are the only phases

observed. However, these oxides were not detected at this temperature in the XRD

pattern of figure 2 in which only unoxidized phases and low intensity peaks

corresponding to TiO2 were identified. These differences are due to the penetration

depth of the X-ray and the fact that both of the cobalt oxides must be located precisely

at the outermost layer of the oxide scale, preventing the detection of other inwardly

located phases. At 900 ºC, CoO appears as the major phase because of the instability of

Co3O4, according to reaction (1), although the marked (200) preferred orientation

observed (2 ≈ 42.4º) could also be at the origin of this predominance in the XRD

pattern.

243 OCoO6OCo2 (1)

Note that the presence of external CoO and Co3O4 implies the migration of cobalt

through the TiO2 oxide layer, which is supposed to be the main product of oxidation,

towards the surface of cermets and its subsequent oxidation. The formation of an

outermost region of nearly pure cobalt oxide has been frequently observed during the

oxidation of Co-base alloys [20, 21].

8

At higher temperatures (1000 ºC and 1100 ºC) (figure 4), when the oxidation has

significantly progressed, TiO2 emerges at the surface of the cermet, while cobalt oxides

disappear by reacting with TiO2 rutile to form cobalt methatitanate (CoTiO3) or cobalt

orthotitanate (Co2TiO4, cubic structure, Fd-3m, 227), according to reactions (2-5), as

follows [22, 23]:

32 CoTiOTiOCoO (2)

23243 OCoTiO6TiO6OCo2 (3)

422 TiOCoTiOCoO2 (4)

242243 OTiOCo3TiO3OCo2 (5)

It is possible to consider the formation of Co2TiO4 from CoTiO3, according to the

following reaction (6):

423 TiOCoCoTiOCoO (6)

The XRD patterns of oxidized surfaces of Ti99Ta1 cermet (figure 4) are generally

similar to those corresponding to Ti100 cermet (with the same shift in temperature as

previously observed in figure 2), although certain differences are also noticed. The first

difference is the presence of rutile and small reflections of cobalt titanates (CoTiO3 and

Co2TiO4) at 800 ºC. For Ti99Ta1 cermet, cobalt oxides dominate the XRD pattern of

the surface at higher temperatures, over the 900-1000 ºC temperature range. Second, the

XRD pattern of the oxidized surface at 1100 ºC shows the presence of intense

reflections of Co2TiO4 and CoTi2O5 (orthorhombic structure, Cmcm, 63). This result is

in contrast to the pattern for Ti100 cermet at this temperature in which only TiO2 was

detected. According to the binary CoO/Co3O4-TiO2 phase diagram [22], cobalt

dititanate (CoTi2O5) is stable at temperatures above ~1140 ºC, and its formation at these

temperatures can occur by reactions (7-10), as follows:

5223 OCoTiTiOCoTiO (7)

2522 OOCoTiTiO2CoO (8)

CoOOCoTiCoTiO2 523 (9)

9

52422 OCoTi2TiOCoTiO3 (10)

The existence of a higher proportion of cobalt titanates, especially Co2TiO4, at

lower temperatures becomes more apparent with increasing Ta content (Ti95Ta5,

Ti90Ta10 and Ti80Ta20 in figure 4). These observations suggest that the substitution

of Ti4+ by Ta5+ in the rutile phase and the formation of Ti3+ in the structure for charge

compensation [24] must increase the reactivity of this phase and promote the formation

of cobalt titanates at lower temperatures through reactions (2-5). Moreover, note that

cobalt oxides (CoO and Co3O4) were not observed in the oxidized surfaces of Ti90Ta10

and Ti80Ta20 cermets over the entire temperature range studied. For both of the

cermets, only the rutile, Co2TiO4 and CoTiO3 phases are observed. It is then possible to

propose for these cermets the direct reaction between cobalt and rutile to produce cobalt

titanates at the surface through reactions (11 and 12) instead of reactions (2-5). The

stoichiometries proposed in reactions (11 and 12) are based on the XEDS results by

SEM (shown later), showing that cobalt titanates do not contain Ta and revealing the

existence in the same specimen of a TixTa1-xO2 solid solution with different Ti/Ta ratios.

213221 OTaTiCoTiOO21OTaTiCo yyxx (11)

2142221 OTaTiTiOCoOOTaTi2Co yyxx (12)

The XRD pattern in figure 4 for the Ti80Ta20 cermet at 1100 ºC only shows the

presence of Co2TiO4 and CoTiO3. Maintaining these cobalt titanates as an outermost

layer in the oxide scale at higher temperatures should be related to the improvement in

oxidation resistance of cermets and would indicate a more protective effect than that

provided by the rutile layer only. Furthermore, it has been shown that Ta5+ doping in the

rutile phase decreases the oxygen vacancy concentration, hindering the oxygen mass

transport [25] through the oxide scale, which should also decelerate the oxidation of

cermets [26].

Finally, note that except for Ti80Ta20 oxidized at 800 ºC, the XRD patterns of

oxidized surfaces for Ti90Ta10 and Ti80Ta20 cermets do not show the presence of

unoxidized phases, although TG measurements evidenced a low degree of oxidation.

This fact is related again to the penetration depth of the X-rays that as confirmed by

10

SEM observations (figure 5) was about 10 m in the present case; thus, the phases

present below this depth are unlikely to be detected.

3.3. Characterization of oxides and subsurfaces in cross-sections by SEM.

To study how oxygen penetrates cermets and to study the distribution and

composition of phases from the external surface of the oxide scale to the inner part of

the different specimens, polished cross-sections of oxidized cermets were analyzed by

SEM. Considering the large number of samples and the wide range of oxidation

temperatures studied, only the more representative results are presented in this section.

Figure 5 shows an example of the cross-sectional SEM images of the different

cermets oxidized at 800 ºC, clearly showing the drastic reduction in oxidation depth as

the Ta content is increased from 220 μm for Ti100 to 6 μm for Ti80Ta20 (note the

different magnification in the SEM images). This result confirms the outstanding

improvement of oxidation resistance when Ta is introduced into the structure of the

carbonitride phase. The same trend was observed with increasing temperature, although

for the same Ta content, the volume affected by oxidation also increases with

temperature (figure 6).

The SEM images in figures 5 and 6 show that the volume affected by oxidation

can be divided into two primary regions, which is more clearly visible in specimens

with a higher oxidation. For example, in Ti100 cermet oxidized at 800ºC, a large

external zone, approximately 147 m wide, only composed of oxidized phases (as

confirmed by XEDS-SEM mapping) is observed (figure 7). The large pores observed in

this region were presumably generated by coalescence of smaller pores formed because

of the volatile species arising from the oxidation of the carbonitride phase. Immediately

below this region, there is a partially oxidized internal degradation zone with lower

porosity. The width of this layer (~ 73 m) and its depth imply an important diffusion of

oxygen into the cermet, which suggests a limited protection of the formed oxide scale.

When Ta is introduced into cermets, both of the regions experienced a significant

reduction in thickness, especially the internal degradation zone in cermets with a higher

Ta content. Simultaneously, the porosity of the fully oxidized external layer was

drastically reduced with the Ta content, which may contribute to increased oxidation

protection.

11

Analytical measurements using XEDS-SEM performed at different locations

(marked with numbers in figures 8-10) were obtained to gain additional information for

the identification of the different phases present in the oxide scale and the internal

degradation zone of oxidized specimens. Representative semi-quantitative compositions

obtained from XEDS-SEM spectra (some characteristic ones are displayed in figure 11)

are shown in table 3 and the calculated Ti/Ta/Co/O atomic ratios were used to assign the

different phases previously detected by XRD to the different areas analyzed. So, the

formation at low temperatures of a thin external layer composed of CoO and Co3O4 was

confirmed for the Ti100 cermet (numbers 1 and 2, respectively, in figure 8a,

corresponding to the outermost zone of Ti100 cermet oxidized at 800 ºC). Immediately

below this layer, the TiO2 phase was detected (number 3 in figure 8a), along with

CoTiO3 (number 4 in the same figure 8a). The irregular morphology present in this

phase and the location of CoTiO3 between cobalt and titanium oxides support its

formation according to reactions (2) and (3). Subsequently, a Co-depleted zone was

detected, and further from the surface, CoTiO3 was again detected (figure 8a). In this

case, this phase could have been directly formed through reaction (11).

At a higher temperature (1000 ºC), when the oxidation has significantly

progressed, the SEM image of the external oxidation zone of Ti100 cermet reveals the

absence of cobalt oxides at the surface and the emergence of TiO2 (number 3 in figure

8b). The existence of CoTiO3 is also observed in figure 8b (number 4) because of

reactions (2) and (3) that would have consumed cobalt oxides formed in an earlier

oxidation stage. Note that although Co2TiO4 was assigned in the superficial XRD

pattern (figure 3), it could not be detected by XEDS-SEM. The presumably low amount

of this phase, the fine scale of the microstructure and the interference from neighboring

phases can cause difficulty in distinguishing between the CoTiO3 and Co2TiO4 phases.

For Ti99Ta1 cermet, a similar phase distribution to that for Ti100 was found in

the superficial oxidation zone. The formation of the external cobalt oxide layer

(numbers 1 and 2 in figure 8c) was confirmed at 800 ºC and was still observed at 1000

ºC (figure 8d); this layer only disappeared at 1100 ºC (figure 8e). At this temperature,

the rutile phase (number 3) emerges at the surface of the oxidized cermet. CoTiO3 was

also detected near the cobalt oxide layer at the lower oxidation temperature, and

Co2TiO4 and CoTi2O5 were evidenced at a high temperature (1100 ºC) (numbers 5 and

6, respectively, in figure 8e).

12

The trend to form cobalt titanates instead of cobalt oxides at low oxidation

temperatures when Ti is increasingly substituted by Ta is illustrated for Ti95Ta5 cermet

in figure 9a. The chemical compositions determined by XEDS-SEM in the outermost

layer of the oxide scale formed at 900 ºC (numbers 4 and 5 in figure 9a) were in

agreement with the XRD results (figure 4), in which the CoTiO3 and Co2TiO4 phases

were detected in the surface of oxidized Ti95Ta5 cermet. At the highest oxidation

temperature, 1100 ºC, the cobalt titanate layer (CoTiO3 and Co2TiO4) remained at the

surface (figure 9b). At this temperature, a small external layer of CoO was observed

(number 1 in figure 9b) in some specific areas. The cobalt titanate layer was only 10

microns thick, and it was continuous and dense; the emergence of the rutile phase at the

external surface of the cermet was never demonstrated.

For Ti90Ta10 and Ti80Ta20 cermets, it was confirmed that a cobalt titanate layer

was always formed and was retained at the surface over the entire oxidation temperature

range studied (800-1100 ºC), as exemplified in figures 9c and 9d for Ti90Ta10 cermet

oxidized at 800 ºC and Ti80Ta20 cermet at 1000 ºC, respectively. This cobalt titanate

layer in the Ti90Ta10 and Ti80Ta20 cermets appears to be composed of the

superposition of two continuous oxides. Co2TiO4 was observed in the outermost zone,

and CoTiO3 appeared immediately below it (numbers 5 and 4, respectively, in figure

9d). The complete absence of cobalt oxides (CoO and Co3O4) at each oxidation

temperature confirms that titanates were formed directly between Co and TixTa1-xO2

according to reactions (11) and (12). However, the thickness of this titanate layer

increased with the oxidation temperature because of the oxidation progress but only

slightly, as shown in figures 9c and 9e for Ti90Ta10 cermet oxidized at 800 ºC and

1000 ºC, respectively. Under this titanate top layer, a TixTa1-xO2 sublayer was observed

(number 3 in figure 9). This layer was thin due to the small degree of oxidation, thus

preventing the accumulation of compressive stresses and avoid the cracking of the

protective titanate layer, which maintains its integrity by remaining dense and

continuous.

Note that the presence of Ta in the rutile phase was ascertained by XEDS-SEM,

confirming the formation of a TixTa1-xO2 solid solution, whereas Ta was never observed

in any of the titanate phases. This result appears to indicate that it is impossible to form

a solid solution for these mixed oxides, probably due to the different structures and

13

compositions of cobalt tantalates (CoTa2O6 and Co4Ta2O9, according to the PFD-ICDD

database) compared with cobalt titanates.

Moreover, note that XEDS-SEM measurements revealed the existence of a

chemical gradient in the TixTa1-xO2 solid solution, which becomes more apparent with

increasing Ta content. A composition virtually without Ta was observed near the

surface, while moving inwards, the amount of Ta increased in the solid solution. As an

example, in figure 9b corresponding to the Ti80Ta20 cermet oxidized at 800 ºC, the

numbers 3a, 3b and 3c represent the TixTa1-xO2 solid solution but with three different

Ti/Ta atomic ratios of ~ 16, 5.3 and 2.4, respectively, determined from the Ti and Ta

atomic percentages obtained from XEDS-SEM analysis. Note that this last ratio is even

lower than the starting nominal ratio of 4 in the carbonitride phase (Ti0.8Ta0.2C0.5N0.5).

These results account for a faster diffusion of Ti toward the external surface to be

oxidized compared with Ta, which could also explain the absence of Ta in cobalt

titanates.

The chemical characterization of the internal degradation zone of the different

oxidized cermets confirmed that it was always composed of a continuous rutile matrix

in which isolated rounded packets of a Co alloy are embedded. As previously

mentioned, this area was dramatically reduced as the Ta content increased and was

extremely small and difficult to observe for cermets with the highest Ta content at any

of the oxidation temperatures studied. Figures 10a, 10b and 10c show this zone for

cermets Ti100 oxidized at 1000 ºC, Ti99Ta1 at 1000 ºC and Ti95Ta5 at 1100 ºC,

respectively.

Semiquantitative analysis by XEDS-SEM performed in the Co alloy pockets

(number 7 in figures 10a, 10b and 10c) always revealed a Co content higher than 90 at%

(Ti and Ta were the other metallic elements detected in the alloy). Because the original

binder phases had a markedly higher Ti and Ta content (see ref. [19]), these XEDS-

SEM analyses confirm that a preferential oxidation of Ti and Ta occurred in the binder

phase.

Finally, after the internal degradation zone and immediately before reaching the

unoxidized phases, a new narrow layer that corresponds to the oxidation front was

observed. In this region, the incipient oxidation of the cermet occurs, and phases in the

Ti-Ta-Co-O system were detected with chemical compositions close to the ceramic and

14

binder phases but with a non-negligible amount of oxygen. As an example, in figure

10d, the oxidation front in the Ti100 cermet oxidized at 900 ºC is shown. In figures 5-7

the oxidation front in oxidized cermets can also be observed.

Figure 10e shows details of the oxidation front of the Ti80Ta20 cermet oxidized

at 1100 ºC. This image and XEDS-SEM analyses performed at different points of the

reaction front allowed the conclusion that the onset and advancement of the oxidation

reaction occurs at the level of the binder phase by the preferential oxidation of Ti and

Ta. Subsequently, oxidation proceeds towards the ceramic particles, leading to the

characteristic microstructure of the internal degradation zone, with Co as the last

element to be oxidized in cermets.

4. Conclusions.

1. The substitution of Ti by Ta in titanium carbonitride-based cermets results in an

extraordinary improvement of the oxidation resistance. This improvement was observed

over the entire temperature range studied (800 ºC-1100 ºC). The improvement was

especially remarkable in Ti80Ta20 cermet, for which oxidation was reduced by more

than 90%.

2. The oxidized zone can be divided in three regions, as follows: a) an external oxide

scale in which fully oxidized phases coexist, such as cobalt oxides (CoO and Co3O4),

titanium-tantalum oxide with the rutile-type structure (TixTa1-xO2) and cobalt titanates

(CoTiO3, Co2TiO4 and CoTi2O5), depending on the temperature and Ta content; b) a

partially oxidized internal degradation zone in which TixTa1-xO2 and -Co coexist; and

c) the oxidation front in which the incipient oxidation of the ceramic and binder phases

occurs. The oxide scale and the internal degradation zones are significantly reduced

when the Ta content is increased in cermets.

3. For the cermets with zero or low Ta content, Ti100 and Ti99Ta1, the degree of

oxidation is important, even at low temperatures, and the formation in the oxide scale of

a thin CoO/Co3O4 external layer is observed. When the oxidation temperature is

increased, these cobalt oxides react with the underlying rutile phase to form cobalt

titanates. The cracking of the CoO/Co3O4 external layer induces the emergence of the

rutile phase at the surface of the oxide scale.

15

4. For the cermets with higher Ta content, Ti95Ta5, Ti90Ta10 and Ti80Ta20, the

formation of a cobalt titanate external layer instead of the CoO/Co3O4 layer is observed,

even at the lowest oxidation temperature studied. This external layer remains stable at

increasing oxidation temperatures.

5. The improved oxidation resistance observed with Ta substitution arises from a

synergic effect that includes the lower tendency of Ta to oxidize, the formation of a

continuous and compact protective layer of cobalt titanate at the surface of the oxide

scale and the formation of a TixTa1-xO2 solid solution with rutile-type structure in which

some Ti4+ are replaced by Ta5+, which reduces oxygen vacancies and then the oxygen

diffusion into cermets.

Acknowledgments.

This study was supported by the Spanish Government under grant MAT2011-

22981, financed in part by the European Regional Development Fund of 2007-2013. E.

Chicardi and J. M. Córdoba were supported by CSIC through the JAE-Pre and JAE-Doc

grants, respectively, which are financed, in part, by the European Social Fund (ESF).

References.

[1] J. Nicholls, Advances in coating design for high-performance gas turbines, Mrs Bulletin, 28 (2003) 659-670. [2] K.J.A. Brookes, Hardmetals and Other Hard Materials, International Carbide Data, 1992. [3] W.B. Eisen, B.L. Ferguson, R.M. German, R. Iacocca, P.W. Lee, D. Madan, K. Moyer, H. Sanderow, Y. Trudel, ASM Handbook Volume 7: Powder Metal Technologies and Applications, ASM International, 1998. [4] J. Pujana, L. del Campo, R.B. Perez-Saez, M.J. Tello, I. Gallego, P.J. Arrazola, Radiation thermometry applied to temperature measurement in the cutting process, Measurement Science & Technology, 18 (2007) 3409-3416. [5] M. Naidoo, O. Johnson, I. Sigalas, M. Herrmann, Influence of tantalum on the microstructure and properties of Ti(C,N)-Ni cermets, International Journal of Refractory Metals & Hard Materials, 42 (2014) 97-102. [6] J.M. Córdoba, M.A. Avilés, M.J. Sayagués, M.D. Alcalá, F.J. Gotor, Synthesis of complex carbonitride powders TiyMT1-yCxN1-x (MT:Zr,V,Ta,Hf) via a mechanically induced self-sustaining reaction, Journal of Alloys and Compounds, 482 (2009) 349-355. [7] U. Rolander, G. Weinl, M. Zwinkels, Effect of Ta on structure and mechanical properties of (Ti,Ta,W)(C,N)-Co cermets, Int. J. Refract. Met. Hard Mat., 19 (2001) 325-328. [8] S. Zhang, Titanium carbonitride-based cermets: processes and properties, Materials Science and Engineering A, 163 (1993) 141-148.

16

[9] P. Ettmayer, H. Kolaska, W. Lengauer, K. Dreyer, Ti(C,N) Cermets - Metallurgy and Properties, International Journal of Refractory Metals and Hard Materials, 13 (1995) 343. [10] H. Pastor, Titanium-carbonitride-based hard alloys for cutting tools, Materials Science and Engineering, 105-106 (1988) 401-409. [11] S. Bolognini, G. Feusier, D. Mari, T. Viatte, W. Benoit, High temperature mechanical behaviour of Ti(C,N)-Mo-Co cermets, International Journal of Refractory Metals & Hard Materials, 16 (1998) 257-268. [12] C. Mroz, Titanium diboride, American Ceramic Society Bulletin, 74 (1995) 158-159. [13] C. Martin, B. Cales, P. Vivier, P. Mathieu, Electrical-discharge machinable ceramic composites, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 109 (1989) 351-356. [14] V.J. Tennery, C.B. Finch, C.S. Yust, G.W. Clark, Science of Hard Materials, Plenum, New York, 1983. [15] R.A. Alliegro, Encyclopedia of electrochemistry, Reinhold, New York, 1964. [16] B.V.M. Kumar, J.R. Kumar, B. Basu, Crater wear mechanisms of TiCN-Ni-WC cermets during dry machining, International Journal of Refractory Metals & Hard Materials, 25 (2007) 392-399. [17] E. Chicardi, F.J. Gotor, J.M. Cordoba, Enhanced oxidation resistance of Ti(C,N)-based cermets containing Ta, Corrosion Science, 84 (2014) 11-20. [18] N.A. Abukhshim, P.T. Mativenga, M.A. Sheikh, Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining, International Journal of Machine Tools and Manufacture, 46 (2006) 782-800. [19] E. Chicardi, Y. Torres, J.M. Cordoba, P. Hvizdos, F.J. Gotor, Effect of tantalum content on the microstructure and mechanical behavior of cermets based on (TixTa1-x)(C0.5N0.5) solid solutions, Materials & Design, 53 (2014) 435-444. [20] L. Klein, A. Bauer, S. Neumeier, M. Goeken, S. Virtanen, High temperature oxidation of gamma/gamma '-strengthened Co-base superalloys, Corrosion Science, 53 (2011) 2027-2034. [21] Y. Niu, F. Gesmundo, F. Viani, F. Rizzo, M.J. Monteiro, The corrosion of two Co-Nb alloys under 1 ATM O-2 at 600-800 degrees C, Corrosion Science, 38 (1996) 193-211. [22] K.T. Jacob, G. Rajitha, Role of entropy in the stability of cobalt titanates, Journal of Chemical Thermodynamics, 42 (2010) 879-885. [23] A. Przepiera, J. Plaska, K. Przepiera, M. Jablonski, A. Konratowska, Preparation of cobalt titanates via co-precipitation while using industrial intermediates as titanium precursors, Polish Journal of Chemical Technology, 11 (2009) 51-54. [24] T. Yamamoto, T. Ohno, Screened hybrid density functional study on Nb- and Ta-doped TiO2, Physical Review B, 85 (2012). [25] V. Guidi, M.C. Carotta, M. Ferroni, G. Martinelli, M. Sacerdoti, Effect of dopants on grain coalescence and oxygen mobility in nanostructured titania anatase and rutile, Journal of Physical Chemistry B, 107 (2003) 120-124. [26] M. Pfeiler, C. Scheu, H. Hutter, J. Schnoeller, C. Michotte, C. Mitterer, M. Kathrein, On the effect of Ta on improved oxidation resistance of Ti-Al-Ta-N coatings, Journal of Vacuum Science & Technology A, 27 (2009) 554-560.

17

FIGURE CAPTIONS.

Figure 1. Weight gain per unit area after 48 h of oxidation for all of the cermets and

oxidation temperatures.

Figure 2. XRD patterns of oxidized cermets reduced to powder after the 48 h oxidation

tests. (a) 800 ºC; (b) 900 ºC; (c) 1000 ºC; (d) 1100 ºC. Phases: () Unoxidized ceramic

phase: TixTa1-xC0.5N0.5; () Unoxidized binder phase: TixTa1-xCo; (▼) Unoxidized

binder phase: TixTa1-xCo2; () TixTa1-xO2 with rutile structure; () CoTiO3; () -Co.

Figure 3. XRD patterns of oxidized cermets reduced to powder after the 48 h oxidation

tests at 1100 ºC over the range of (110) and (211) reflections for TiO2 and TaO2 with

the rutile structure. a) Ti100; b) Ti99Ta1; c) Ti95Ta5; d) Ti90Ta10; and e) Ti80Ta20.

Figure 4. XRD patterns of the surface of the oxidized cermets after the 48 h oxidation

tests. (a) 800 ºC; (b) 900 ºC; (c) 1000 ºC; (d) 1100 ºC. Phases: () Unoxidized ceramic

phase TixTa1-xC0.5N0.5; (o) Co3O4; (Δ) CoO; () TixTa1-xO2 with rutile structure; ()

CoTiO3; (+) Co2TiO4; () CoTi2O5.

Figure 5. Cross-sectional SEM images of the different cermets oxidized at 800 ºC,

showing the approximate measurements of the oxidation penetration depth. The dotted

lines separate the completed oxidation zones, the internal degradation zones and the

unoxidized cermet areas.

Figure 6. Cross-sectional SEM images of the cermet Ti95Ta5 oxidized at 800 ºC, 900

ºC, 1000 ºC and 1100 ºC, showing the approximate measurements of the oxidation

penetration depth. The dotted lines separate the completed oxidation zones, the internal

degradation zones and the unoxidized cermet areas.

Figure 7. A characteristic XEDS-SEM mapping of Ti100 cermet oxidized at 800ºC,

corroborating the clear differentiation between the oxidized / unoxidized phases and

used to measure the oxidation penetration depth showed in figures 5 and 6.

Figure 8. Representative cross-sectional SEM images of cermets Ti100 and Ti99Ta1

showing the oxidized phases in the completed oxidation zone as determined by XEDS-

SEM. a) Ti100 at 800 ºC; b) Ti100 at 1000 ºC; c) Ti99Ta1 at 800 ºC; d) Ti99Ta1 at

18

1000 ºC; e) Ti99Ta1 at 1100 ºC. (1) CoO; (2) Co3O4; (3) TiO2; (4) CoTiO3; (5)

Co2TiO4; (6) CoTi2O5.

Figure 9. Representative cross-sectional SEM images of cermets Ti95Ta5, Ti90Ta10

and Ti80Ta20 showing the phases in the completed oxidation zone as determined by

XEDS-SEM. a) Ti95Ta5 at 900 ºC; b) Ti99Ta5 at 1100 ºC; c) Ti90Ta10 at 800 ºC; d)

Ti80Ta20 at 1000 ºC; e) Ti90Ta10 at 1000 ºC. (1) CoO; (3a), (3b) and (3c) TixTa1-xO2;

(4) CoTiO3; (5) Co2TiO4.

Figure10. Representative cross-sectional SEM images of the internal degradation zone

[a) Ti100 at 1000 ºC; b) Ti99Ta1 at 1000 ºC; c) Ti95Ta5 at 1100 ºC] and the oxidation

front [d) Ti100 at 900 ºC; e) Ti80Ta20 at 1100 ºC] for the different oxidized cermets,

showing the phases in these regions as determined by XEDS-SEM. (3) TixTa1-xO2 (7) -

Co.

Figure 11. Representative semi-quantitative XEDS-SEM spectra of oxidized phases

found in the different areas analyzed and previously detected by XRD.

19

TABLES.

Table 1. Percentage reduction of the weight gain per unit area for each cermet at each

oxidation temperature compared to the weight gain per unit area of the Ti100 cermet

(reference material) after the 48h oxidation tests.

Cermet 800ºC 900ºC 1000ºC 1100ºC

Ti100 - - - -

Ti99Ta1 67.2 67.2 48.0 46.3

Ti95Ta5 89.2 90.7 84.9 72.8

Ti90Ta10 94.1 94.2 89.3 85.6

Ti80Ta20 95.6 96.3 94.0 92.8

20

Table 2. Lattice parameters for the TixTa1-xO2 solid solution with rutile-type structure

determined from the X-ray diffractograms of cermets oxidized at 1100ºC.

Cermet a=b (Å) c (Å)

Ti100 4.5933 2.9592

Ti99Ta1 4.5935 2.9608

Ti95Ta5 4.6025 2.9666

Ti90Ta10 4.6139 2.9761

Ti80Ta20 4.6314 2.9823

21

Table 3. Punctual semi-quantitave compositions calculated from XEDS-SEM spectra

performed in different zones of the oxide scale and internal oxidation region of the

oxidized cermets. The numbers in figures 8-10 mark the analyzed zones. The nature of

phases that best suits the results of the analyses is also shown.

Zone Ti (at%) Ta (at%) Co (at%) O (at%) Approximate phase

stoichiometry

1 - - 50.4 49.6 CoO

2 - - 58.4 41.6 Co3O4

3 34.3 - - 65.7 TiO2

3a 28.1 1.7 - 70.2

TixTa1-xO2 3b 25.5 4.8 - 69.7

3c 20.8 8.8 - 70.4

4 24.7 - 22.7 52.6 CoTiO3

5 15.8 - 29.6 54.6 Co2TiO4

6 25.4 - 13.1 61.5 CoTi2O5

7 3.9 1.3 94.8 - α-Co

0 5 10 15 20 25 30 35

0

20

40

60

80

100

120

140

Ti80Ta20Ti90Ta10

Ti95Ta5

Ti99Ta1

Ti100

Ta (wt.%)

m

48 (

mg

/cm

2)

800ºC

900ºC

1000ºC

1100ºC

Figure 1

20 30 40 50 60 70 80

Ti90Ta10I

(ncp

s)

2 (º)

·

· ·

· ·

· ·

·

· ·

· ·

·

· ·

· ·

·

· ·

(d)

(c)

(b)

(a)

20 30 40 50 60 70 80

* · ©

©

Ti100

(d)

(c)

(b)

(a)

©

·

· ·

·

¨

·

©

©

* * * * *

* *

¨

· ·

·

·

·

©

2 (º)

I (n

cps)

20 30 40 50 60 70 80

Ti99Ta1

I (n

cps)

2 (º)

*

·

¨· ·

· ·

·

* *

·

·

·

¨

©

©

¨

·

·

·

¨

·

·

· ©¨

·

(d)

(c)

(b)

(a)

20 30 40 50 60 70 80

©

I (n

cps)

2 (º)

Ti95Ta5

· ·

¨

¨

¨

·

·

·

·

·

· ·

·

·

· ·

· ·

· ·

· · ·

(d)

(c)

(b)

(a)

20 30 40 50 60 70 80

Ti80Ta20

· ·

·

· ·

· ·

·

· ·

· · ·

· ·

· ·

·

· ·

I (n

cps)

2 (º)

(d)

(c)

(b)

(a)

Figure 2

Figure 3

52 53 54 55 56 57

TiO2

2 (º)I

(ncp

s)

TaO2

e)

d)

c)

b)

a)

(211)

25 26 27 28 29 30

e)

d)

c)

b)

a)

(110) TaO

2 TiO

2

2 (º)

I (n

cps)

20 30 40 50 60 70 80

*

+

2 (º)

I (n

cps)

Ti100

* *

+

(a)

(d)

(c)

(b)

20 30 40 50 60 70 80

I (n

cps)

2 (º)

Ti99Ta1

+

+

+

+

+

+

+

*

+

(a)

(d)

(c)

(b)

20 30 40 50 60 70 80

+

*

I (n

cps)

2 (º)

Ti95Ta5

**

+

*

*

+

**+

++

+

+

*

+

+

+

+

+

+

+

+

*

+

+

+

+

+

+

++

*

+*

**

+

+

(a)

(d)

(c)

(b)

20 30 40 50 60 70 80

Ti90Ta10

I (n

cps)

2 (º)

*

+

+

+

+

*

**

+

+

*

+

*

+

+

+

*

++

+

*

*

*

*

+

+

+

· · ·

+

+

*

**

*

(a)

(d)

(c)

(b)

20 30 40 50 60 70 80

*

I (n

cps)

2 (º)

*

*

*

*

Ti80Ta20

+

+

+

+

*

*

*

*

· ·

**+

+

+

+

*

*

*

+

+

+

+

+

+

*

+

+

+

+

+

+

+

*

*

*

*

· ·

*

*

(a)

(d)

(c)

(b)

Figure 4

Ti80Ta20

5 μm

6 μm

Figure 5

100 μm

220 μm

Ti100

147 μm

20 μm

Ti99Ta1

56 μm 49 μm

20 μm

18 μm Ti95Ta5

12 μm Ti90Ta10

10 μm

11 μm 9 μm

800 ºC 900 ºC

1000 ºC 1100 ºC

20 μm

200 μm

18 μm

10 μm

45 μm

158 μm

50 μm

470 μm

Figure 6

12 μm 35 μm

93 μm 283 μm

90 μm

O

Co Ti

Figure 7

Figure 8

10 μm 20 μm

20 μm 20 μm

1 2

3 4

4

4

3

3

1 2 2

1

3

a) b)

c) d)

50 μm

3

5

5

6

e)

Figure 9

4

a)

3

5

5 5

10 μm

5

b)

4

1

10 μm 3

10 μm

c) 5

3

20 μm

d) 5 4

3a

a 3c

3b

10 μm

e) 5

3

Figure 10

a)

20 μm

7

20 μm

20 μm

2 μm

7

b)

c) d)

e)

3

7

7

3

7 7

3

20 μm

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CoO

OK

CoL

CoK

I (n

cps)

E (keV)0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Co3O

4

CoK

CoL

OK

I (n

cps)

E (keV)

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

TixTa

1-xO

2

TaL

TaM

TiK

I (n

cp

s)

E (keV)

OK

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Co2TiO

4

OK

·

· TiL

CoL

TiK

CoKI

(ncp

s)

E (keV)

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

CoTi2O

5

OKI

(ncp

s)

E (keV)

CoL·

TiK

CoK

· TiL

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

·

CoTiO3

I (n

cp

s)

E (keV)

OK

· TiL

CoL

CoK

TiK

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-Co

TiK

CoK

CoL

I (n

cps)

E (keV)

0 2 4 6 8 10 12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

TiO2

·

OK

· TiL

TiK

I (n

cp

s)

E (keV)

Figure 11